The invention relates to polyurethane coatings which may be prepared, e.g., by the casting or knife coating process on flexible substrates such as textile or leather by means of reactive compositions (reactive binders) based on isocyanate-containing compounds and polyols.
Solvent-free and low-solvent reactive compositions based on NCO prepolymers for the preparation of polyurethane coatings on flexible substrates are well known. For example, a process for the preparation of polyurethane coatings is described in EP-A 0 490 158 which is characterised by high reactivity. In view of its high reactivity, such a process is suitable only for spray applications and may not be used in the casting or knife coating process due to its very short pot life. Systems based on blocked polyisocyanates, on the other hand, are suitable for such coating processes, such as, e.g., the combination of blocked NCO prepolymers and cycloaliphatic diamines. In this case a ketoxime-blocked NCO prepolymer based on aromatic polyisocyanates is mixed with a diamine as chain extender and then cured by a heat treatment with cleavage of the ketoxime and optionally evaporation of the solvent. Reactive compositions of this kind for coating flexible substrates are described, e.g., in DE-A 2 902 090. When the systems described are cured, cleavage of the ketoxime therefore takes place, entailing a waste air treatment even when the systems contain no solvents.
EP-A 0 825 209, on the other hand, describes reactive binders free from cleavage products which, in view of their favourable reaction kinetics and very good mechanical film properties, are outstandingly suitable for coating textiles and other flat substrates. A disadvantage is that the preparation of reactive binders with a low monomer content is technically more complex (monomer separation by thin film distillation).
The object of the present invention was, therefore, to provide polyurethane coatings which may be obtained from reactive binders with a low monomer content which can be prepared without monomer separation.
Surprisingly, by combining certain polyisocyanate prepolymers containing uretidione or oxadiazine trione groups and certain diol chain extenders, it is possible to obtain coatings with good mechanical properties which meet the requirements for the coatings of flexible substrates such as, for example, textile, and therefore achieve this object.
The invention provides, therefore, polyurethane coatings which are characterised in that they may be obtained by reaction of aliphatic, isocyanate-containing prepolymers A) with a polyol component B), wherein
the prepolymers A)
based on solids, contain 1-8 wt. % of NCO, have a free monomeric diisocyanate content of less than 2% and contain from 0.1 to 1.5 mole of aliphatically bound uretidione (I) and/or oxadiazine trione groups (II) per 1000 g of prepolymer 
and the polyol component B)
is composed of at least 50 equivalent %, based on the groups which are reactive towards isocyanate, of compounds corresponding to the formula
HOxe2x80x94Xxe2x80x94Yxe2x80x94Xxe2x80x94OHxe2x80x83xe2x80x83(III)
wherein
Y means methylene, ethylene, xe2x80x94Cxe2x89xa1Cxe2x80x94, cyclohexylene-1,4, -1,3, -1,2, phenylene-1,4, -1,3, -1,2 and
X means methylene, xe2x80x94OCH2CH2xe2x80x94 (the oxygen being bound to the Y), cyclohexylene-1,4, -1,3, -1,2,
and
wherein the equivalent ratio of the free NCO groups of A to the NCO-reactive groups of B is 0.90 to 1.50, optionally with the use of conventional auxiliaries and additives, the use thereof for coating flexible substrates such a textile or leather, and processes for coating said substrates, e.g., by casting or knife coating followed by heat treatment, and the coated substrates themselves.